The present invention relates to a method for the preparation of a silicone rubber composition or, more particularly, to a method for the preparation of an organopolysiloxane composition curable to give a cured silicone rubber body having good releasability from the surface of a mold on which the composition has been cured.
Several types of silicone rubber compositions have good releasability from the surface of a mold on which it has been cured so that such a silicone rubber composition is widely and traditionally used to prepare a matrix for mold-making of important historical or monumental materials and masterpieces of formative arts. In recent years, new applications of such silicone rubber compositions are under development in the manufacture of various kinds of industrial articles including parts and components of electric and electronic appliances, automobiles and other machines.
As a material used in the above mentioned mold-making for the industrial manufacture, it is essential that the silicone rubber-made matrix is capable of giving a large number of high-fidelity molded replicas sometimes having a very complicated configuration with high productivity and the molded replica has high mechanical strengths with long serviceable life. Also the silicone rubber-made matrix should have good handleableness and withstand the use in contact with certain synthetic resins which might cause degradation of the silicone rubber.
Various attempts have been made to meet the above mentioned requirements for the silicone rubber compositions used in mold-making. For example, Japanese Patent Publication No. 47-30422 teaches a method in which the silicone rubber composition comprises a hydrophobic silica filler having the surface blocked with triorganosiloxy groups of the unit formula R.sub.3 SiO.sub.0.5, in which R is a monovalent hydrocarbon group, and an organopolysiloxane of which the molecular chain terminals are blocked each with a silanolic hydroxy group. Japanese Patent Publication No. 40-19178 teaches a method in which the silicone rubber composition comprises a reinforcing filler, an organopolysiloxane having silicon-bonded vinyl groups at the molecular chain terminals and a nitrogen-containing organosilicon compound. The silicone rubber compositions obtained by these methods are not always quite satisfactory as a material for mold making in respect of the surface releasability and durability.
In order to comply with the requirements for such a silicone rubber compositions used in mold-making of complicated and exquisite articles such as high tensile strength, ultimate elongation and tear strength, it is proposed that a base silicone rubber composition of the so-called addition-curable type comprising a vinyl-containing organopolysiloxane, organohydrogenpolysiloxane and platinum catalyst is further admixed with a resinous organopolysiloxane such as so-called MQ, MTQ, MDQ and MDTQ resins consisting of a combination of the monofunctional or M units of the formula R.sub.3 SiO.sub.0.5, tetra-functional or Q units of the formula SiO.sub.2, trifunctional or T units of the formula RSiO.sub.1.5 and/or difunctional or D units of the formula R.sub.2 SiO, in which R is typically a monovalent hydrocarbon group. The cured silicone rubber body prepared from such a silicone rubber composition, however, is deficient in the poor tear strength so that the silicone rubber-made mold can hardly be used in the molding of a claviform article. Although the deficiency in the mechanical strengths can be remedied by admixing an increased amount of a reinforcing filler, this measure is accompanied by another disadvantage of a decrease in the surface releasability.